Fluorides in nanoporous, electrically-conductive scaffolding matrix for metal and metal-ion batteries

ABSTRACT

A battery electrode composition is provided that comprises composite particles. Each composite particle may comprise, for example, active fluoride material and a nanoporous, electrically-conductive scaffolding matrix within which the active fluoride material is disposed. The active fluoride material is provided to store and release ions during battery operation. The storing and releasing of the ions may cause a substantial change in volume of the active material. The scaffolding matrix structurally supports the active material, electrically interconnects the active material, and accommodates the changes in volume of the active material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present Application for Patent is a Continuation of U.S. patentapplication Ser. No. 17/022,020, entitled “FLUORIDES IN NANOPOROUS,ELECTRICALLY-CONDUCTIVE SCAFFOLDING MATRIX FOR METAL AND METAL-IONBATTERIES,” filed Sep. 15, 2020, which is a Continuation of U.S. patentapplication Ser. No. 16/292,241, entitled “FLUORIDES IN NANOPOROUS,ELECTRICALLY-CONDUCTIVE SCAFFOLDING MATRIX FOR METAL AND METAL-IONBATTERIES,” filed Mar. 4, 2019, which is a Continuation of U.S. patentapplication Ser. No. 14/553,593, entitled “FLUORIDES IN NANOPOROUS,ELECTRICALLY-CONDUCTIVE SCAFFOLDING MATRIX FOR METAL AND METAL-IONBATTERIES,” filed Nov. 25, 2014, which claims the benefit of U.S.Provisional Application No. 61/910,217, entitled “FLORIDES CONFINEDWITHIN SMALL NANOPORES OF CARBON PARTICLES FOR METAL ION AND METALBATTERIES,” filed Nov. 29, 2013, assigned to the assignee hereof, andeach of which is expressly incorporated herein by reference in itsentirety.

BACKGROUND Field

The present disclosure relates generally to energy storage devices, andmore particularly to metal and metal-ion battery technology and thelike.

Background

Owing in part to their relatively high energy densities, light weight,and potential for long lifetimes, advanced metal-ion batteries such aslithium-ion (Li-ion) batteries are desirable for a wide range ofconsumer electronics. However, despite their increasing commercialprevalence, further development of these batteries is needed,particularly for potential applications in low- or zero-emission,hybrid-electrical or fully-electrical vehicles, consumer electronics,energy-efficient cargo ships and locomotives, aerospace applications,and power grids.

Conversion-type electrodes, such as fluorides, sulfides, oxides,nitrides, phosphides, hydrides and others for Li-ion batteries offerhigh gravimetric and volumetric capacities.

Fluorides, in particular, offer a combination of relatively high averagevoltage and high capacities, but suffer from several limitations forvarious metal-ion (such as Li-ion) battery chemistries. For example,only selected metal fluoride particles have been reported to offer somereasonable cycle stability in Li-ion battery cells (specifically AgF₂,FeF₂, FeF₃, CoF₂, and NiF₂). Many other metal fluorides are commonlybelieved not to be practical for applications in Li-ion batteries due tothe irreversible changes that occur in such cathodes during batteryoperation. For example, during Li-ion insertion into some of the otherfluorides (such as CuF₂, for example) and the subsequent formation ofLiF during the conversion reaction, the original fluoride-formingelement (such as Cu in the case of CuF₂) produces electrically isolated(Cu) nanoparticles. Being electrically isolated, such nanoparticlescannot electrochemically react with LiF to transform back into CuF₂during subsequent Li extraction, thereby preventing reversibility of theconversion reaction. As a result, after a discharge, the cell cannot becharged back to the initial capacity.

However, even the cathodes based on those metal fluorides that arebelieved to be most practical due to their relatively reversibleoperation and reasonably low cost (such as FeF₂, FeF₃, CoF₂, and NiF₂),suffer from multiple limitations including: (i) low electricalconductivity, which limits their utilization and both energy and powercharacteristics in batteries; (ii) low ionic conductivity, which limitstheir utilization and both energy and power characteristics inbatteries; and (iii) volume changes during metal ioninsertion/extraction, which may cause mechanical and electricaldegradation in the electrodes during battery operation. As a result,despite the theoretical advantages of fluoride-based cathodes, forexample, their practical applications in metal-ion batteries aredifficult to achieve. The cells produced with fluoride-based cathodescurrently suffer from poor stability, volume changes, slow charging, andhigh impedance.

Several approaches have been developed to overcome some of theabove-described difficulties, but none have been fully successful inovercoming all of them.

For example, decreasing particle size decreases the ion diffusiondistance, and offers one approach to addressing the low ionicconductivity limitation. However, nanopowders suffer from highelectrical resistance caused by the multiple, highly resistive pointcontacts formed between the individual particles. In addition, smallparticle size increases the specific surface area available forundesirable electrochemical side reactions. Furthermore, simplydecreasing the particle size does not address and may in some casesexacerbate other limitations of such materials, such as volume changesas well as weakening of the particle-binder interfaces. Finally, incontrast to using micron-scale particles for cathode formulations,handling nanoparticles and using them to prepare dense electrodes istechnologically difficult. Nanoparticles are difficult to disperseuniformly within conductive carbon additives and binder of the cathodeand the undesirable formation of agglomerates of nanoparticles tends totake place. Formation of such agglomerates reduces the electrode density(thus reducing volume-normalized capacity and energy density of thecells), reduces electrode stability (since binder and conductiveadditives do not connect individual particles within such agglomerates)and reduces capacity utilization (since some of the nanoparticles becomeelectrically insulated and thus do not participate in Li-ion storage).

In another approach, selected metal fluoride particles which offer somereasonable cycle stability in Li-ion battery cells (specifically FeF₂,FeF₃, CoF₂, and NiF₂) may be mechanically mixed with (in some cases byusing high energy milling, as described, for example, in U.S. Pat. No.7,625,671 B2) or deposited onto the surface of conductive substrates,such as carbon black, graphite, multi-walled carbon nanotubes, or carbonfibers. In this case, the high electrical conductivity of the carbonenhances electrical conductivity of the electrodes. However, the phasetransformations during battery operation and the volume changesdiscussed above may induce the separation of the active material fromthe conductive additives, leading to resistance growth and batterydegradation.

In yet another approach, selected metal fluoride particles (specificallyFeF₂ particles) may be coated with a solid multi-walled graphitic carbonshell layer. In this case, the electrical conductivity of a metalfluoride cathode may be improved. However, the above-described volumechanges during metal ion insertion may break the graphitic carboncoating and induce irreversible capacity losses. Similarly, the phasetransformation during subsequent charging and discharging cycles mayinduce a separation of the active material from the graphitic carbonshell, leading to resistance growth and battery degradation.

Accordingly, there remains a need for improved metal and metal-ionbatteries, components, and other related materials and manufacturingprocesses.

SUMMARY

Embodiments disclosed herein address the above stated needs by providingimproved battery components, improved batteries made therefrom, andmethods of making and using the same.

Battery electrode compositions are provided that comprise compositeparticles. Each composite particle may comprise, for example, activefluoride material and a nanoporous, electrically-conductive scaffoldingmatrix within which the active fluoride material is disposed. The activefluoride material is provided to store and release ions during batteryoperation. The storing and releasing of the ions may cause a substantialchange in volume of the active material. The scaffolding matrixstructurally supports the active material, electrically interconnectsthe active material, and accommodates the changes in volume of theactive material.

The scaffolding matrix may comprise pores that have, for example, anaverage characteristic pore width in the range of about 1 nanometer toabout 10 nanometers. In some designs, the active fluoride material maycomprise a first metal fluoride and a second metal fluoride.

In some designs, each composite particle may further comprise a shell atleast partially encasing the active fluoride material and thescaffolding matrix, the shell being substantially permeable to the ionsstored and released by the active fluoride material. For example, theshell may comprise a protective layer formed from a material that issubstantially impermeable to electrolyte solvent molecules. As anotherexample, the shell may comprise an active material layer, wherein theactive material layer is formed from a different active material thanthe active fluoride material disposed within the scaffolding matrix. Theactive material of the active material layer may have a substantiallylower capacity relative to the active fluoride material. As anotherexample, the shell may comprise a porous layer having a smaller averagepore size than the scaffolding matrix. Pores in the porous layer of theshell may be infiltrated with the same active fluoride material as theactive fluoride material disposed within the scaffolding matrix. Asanother example, the shell may be a composite material comprising aninner layer and an outer layer. The inner layer may be a porous layer,for example, having a smaller average pore size than the scaffoldingmatrix, and the outer layer may be, for example, (i) a protective layerformed from a material that is substantially impermeable to electrolytesolvent molecules or (ii) an active material layer formed from an activematerial that is different from the active material disposed within thescaffolding matrix. As another example, the shell may be a compositematerial comprising two or more materials arranged in aninterpenetrating configuration such that each of the materials of thecomposite material contacts the scaffolding matrix.

In some designs, each composite particle may further comprise externalchannel pores extending from an outer surface of the scaffolding matrixtowards the center of the scaffolding matrix, providing channels fordiffusion of the ions into the active material disposed within thescaffolding matrix. At least some of the external channel pores may befilled with (i) a porous material having a different microstructure thanthe scaffolding matrix, (ii) an active material that is different fromthe active fluoride material disposed within the scaffolding matrix,and/or (iii) a solid electrolyte material.

In some designs, each composite particle may further comprise aprotective material at least partially penetrating the scaffoldingmatrix with a radial-varying composition along a radius of the particle,the protective material being substantially permeable to the ions storedand released by the active fluoride material.

Methods of fabricating a battery electrode composition comprisingcomposite particles are also provided. An example method may comprise,for example, providing an active fluoride material to store and releaseions during battery operation, and forming a nanoporous,electrically-conductive scaffolding matrix within which the activefluoride material is disposed. The storing and releasing of the ions maycause a substantial change in volume of the active material. Thescaffolding matrix may structurally support the active material,electrically interconnect the active material, and accommodate thechanges in volume of the active material.

In some designs, forming the scaffolding matrix may comprise, forexample, forming a carbon-containing precursor, oxidizing andcarbonizing the carbon-containing precursor to form a carbonizedparticle, and activating the carbonized particle at elevated temperatureto form the scaffolding matrix with a pore volume of greater than 50vol. %. In some designs, the active fluoride material-infusedscaffolding matrix may be formed, for example, as a powder comprisingparticles, with the method further comprising mixing the active fluoridematerial-infused scaffolding matrix particles with a binder, and castingthe binder-bonded particles onto a metal foil current collector. In somedesigns, the method may further comprise forming a shell at leastpartially encasing the active fluoride material and the scaffoldingmatrix, the shell being substantially permeable to the ions stored andreleased by the active material. In some designs, at least a portion ofthe shell material may be deposited by chemical vapor deposition. Insome designs, at least an outer portion of the shell may be depositedelectrochemically during one or more initial battery cycles, duringwhich electrochemical decomposition of at least some electrolytecomponents occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are presented to aid in the description ofembodiments of the invention and are provided solely for illustration ofthe embodiments and not limitation thereof.

FIG. 1 illustrates an example battery (e.g., Li-ion) in which thecomponents, materials, methods, and other techniques described herein,or combinations thereof, may be applied according to variousembodiments.

FIG. 2 illustrates a comparison of a voltage profile of Li insertioninto an iron fluoride vs. Li insertion into a copper fluoride.

FIGS. 3A-3B illustrate two particular examples of active fluoridenanoparticles or active fluoride nanoplatelets infiltrated into aconductive scaffolding matrix of a nanoporous material with (A) straightand (B) curved pores. The orientation of the pores is not indicated inthis schematic.

FIGS. 4A-4D illustrate four particular examples of the relative changesin average pore volume of a scaffolding matrix as a function of thedistance from the center of the particle towards the surface of theparticle.

FIG. 5 illustrates a schematic of charge and discharge processes withinan example pore, where Li ions react with metal fluoride nanoclusters toform nanoclusters of a fluoride-forming element and nanoclusters of LiF.

FIGS. 6A-6B illustrate two particular examples of fluoride-containingcomposites, where active fluoride nanoparticles or active fluoridenanoplatelets are infiltrated into a scaffolding matrix of a nanoporousmaterial, and where the overall fluoride-matrix composite particle isfurther coated with a protective layer of electrolytesolvent-impermeable shell material conductive to Li ions.

FIGS. 7A-7B illustrate two particular examples of fluoride-containingcomposites with a protective shell layer being a composite materialaccording to various embodiments.

FIGS. 8A-8B illustrate two particular examples of fluoride-containingcomposites according to various embodiments, where a fluoride-conductivecore contains both relatively small (e.g., smaller than 6 nm) pores toconfine fluorides and relatively large “channel” pores (e.g., greaterthan about 6 nm) designed to provide rapid transport of ions from theelectrolyte solution into the core of the particles.

FIGS. 9A-9D illustrate four particular examples of metal fluoridecontaining composites in various embodiments, where the compositesadditionally comprise intercalation-type active materials or surfaceprotective materials, where the relative mass fraction of such materialsform a gradient from the core to the surface of the particles, and wherea higher concentration of the protective (or intercalation-type active)materials is near the surface of the particles.

FIGS. 10A-10B illustrate two particular examples of an individualfluoride nanoparticle confined within a pore of a conductive scaffoldingmatrix and further coated either with another metal fluoride (which mayprovide advantageous stability within the potential range of operationor improved rate performance) or a protective layer of another material,such as an intercalation-type active material, a solid electrolyte, or amixed conductor.

FIGS. 11A-11B illustrate two particular examples of the relative changesin the average mass fraction of the fluoride as a function of thedistance from the center to the surface of a fluoride-comprisingcomposite particle.

FIGS. 12A-12D illustrate four particular examples of relative changes inthe average mass fraction of a protective material as a function of thedistance from the center to the surface of a fluoride-comprisingcomposite particle.

FIGS. 13A-13D illustrate four particular examples of relative changes inthe average mass fraction of an intercalation-type active material as afunction of the distance from the center to the surface of afluoride-comprising composite particle.

FIGS. 14A-14C illustrate three particular examples of relative changesin the average mass ratio of two metal fluorides (MF1 and MF2) as afunction of the distance from the center to the surface of a compositeparticle comprising two different metal fluoride compositions, where oneof the metal fluorides (e.g., MF1) provides higher energy density andanother one (e.g., MF2) improves stability or rate performance of thecomposite.

FIG. 15 illustrates an example of a building block of a Li-ion batterywith volume and thickness changing electrodes (e.g., changes of overabout 6 vol. % in each) designed to counter-balance changes in batterydimensions during battery operation.

FIG. 16 illustrates a particular example of an electrode composed offluoride infiltrated into a conductive scaffolding matrix of ananoporous material, where the electrode is either further coated with alayer of a solid electrolyte or a solid electrolyte completelyinfiltrates the pores between the individual fluoride-containingcomposite particles.

FIG. 17 illustrates a particular example of an irregularly shaped activeparticle comprising a fluoride infiltrated in a conductive nanoporousscaffolding matrix.

FIGS. 18A-18D illustrate four particular examples utilizing scaffoldparticles with large pores.

FIG. 19 includes a flow chart illustrating an example method offabricating a metal fluoride-containing composite electrode.

FIGS. 20A-20B include a flow chart illustrating another example methodof fabricating a metal fluoride-containing composite electrode.

FIGS. 21A-21B include a flow chart illustrating another example methodof fabricating a metal fluoride-containing composite electrode.

FIGS. 22A-22B include a flow chart illustrating another example methodof fabricating a metal fluoride-containing composite electrode with aprotective coating formed in-situ after battery assembling.

DETAILED DESCRIPTION

Aspects of the present invention are disclosed in the followingdescription and related drawings directed to specific embodiments of theinvention. The term “embodiments of the invention” does not require thatall embodiments of the invention include the discussed feature,advantage, process, or mode of operation, and alternate embodiments maybe devised without departing from the scope of the invention.Additionally, well-known elements of the invention may not be describedin detail or may be omitted so as not to obscure other, more relevantdetails.

While the description below may describe certain examples in the contextof Li and Li-ion batteries (for brevity and convenience, and because ofthe current popularity of Li technology), it will be appreciated thatvarious aspects may be applicable to other rechargeable and primary,metal and metal-ion batteries (such as Na-ion, Mg-ion, and others).Further, while the description below may also describe certain examplesof the material formulations in a Li-free (e.g., charged) state, it willbe appreciated that various aspects may be applicable to Li-containingelectrodes (e.g., in either a partially or fully discharged state).

FIG. 1 illustrates an example metal-ion (e.g., Li-ion) battery in whichthe components, materials, methods, and other techniques describedherein, or combinations thereof, may be applied according to variousembodiments. A cylindrical battery is shown here for illustrationpurposes, but other types of arrangements, including prismatic or pouch(laminate-type) batteries, may also be used as desired. The examplebattery 100 includes a negative anode 102, a positive cathode 103, aseparator 104 interposed between the anode 102 and the cathode 103, anelectrolyte (not shown) impregnating the separator 104, a battery case105, and a sealing member 106 sealing the battery case 105.

Both conventional liquid and solid electrolytes may be used for thedesigns herein. Conventional electrolytes for Li- or Na-based batteriesof this type are generally composed of a single Li or Na salt (such asLiPF₆ for Li-ion batteries and NaPF₆ or NaClO₄ salts for Na-ionbatteries) in a mixture of solvents (such as a mixture of carbonates).The most common salt used in a Li-ion battery electrolyte, for example,is LiPF₆, while less common salts include lithium tetrafluoroborate(LiBF₄), lithium perchlorate (LiClO₄), lithium bis(oxalato)borate(LiB(C₂O₄)₂, lithium difluoro(oxalate)borate (LiBF₂(C₂O₄)), lithiumimides (such as SO₂FN⁻(Li⁺)SO₂F, CF₃SO₂N⁻(Li⁺)SO₂CF₃,CF₃CF₂SO₂N⁻(Li⁺)SO₂CF₃, CF₃CF₂SO₂N⁻(Li⁺)SO₂CF₂CF₃,CF₃SO₂N⁻(Li⁺)SO₂CF₂OCF₃, CF₃OCF₂SO₂N⁻(Li⁺)SO₂CF₂₀CF₃,C₆F₅SO₂N⁻(Li⁺)SO₂CF₃, C₆F₅SO₂N⁻(Li⁺)SO₂C₆F₅ or CF₃SO₂N⁻(Li⁺)SO₂PhCF₃,and others) and others. Electrolytes for Mg-ion, Ca-ion, and Al-ionbatteries are often more exotic as these batteries are in earlier stagesof development. They may comprise different salts and solvents (in somecases, ionic liquids may replace organic solvents for certainapplications).

In addition, solid electrolytes may provide some advantages forfluoride-based cathodes, such as stability against oxidation at highcathode potentials, reduced undesirable side reactions between thecathode and electrolyte, as well as enhanced safety. Examples of thesolid ceramic electrolytes include sulfide-based electrolytes (such asLi₂S—P₂S₅, Li₂S—Ga₂S₃—GeS₂, Li₂S—SiS₂, etc.), oxide-based electrolytes(such as Li-La-Ti—O garnet, Li-La-Ta—O garnet, Li—Si—O glass, Li—Ge—Oglass, Li₉SiAlO₈, etc.), mixed sulfide-oxide electrolytes (such asLi₂S—SiS₂—Li₄SiO₄, LiI—La₂O₂S—La₂O₂S₂, etc.), and many others. The useof solid electrolytes with conversion-based cathodes has been hinderedby the inability of ceramics to accommodate the volume changes that takeplace during charge and discharge cycling. Their use with fluoride-basedcathodes has been particularly difficult because fluoride-based activecathode particles exhibit large volume changes.

Conventional cathode materials utilized in metal-ion batteries are of anintercalation-type. Metal ions are intercalated into and occupy theinterstitial positions of such materials during the discharge of abattery. However, such cathodes exhibit small gravimetric and moreimportantly small volumetric capacities: typically less than 220 mAh/gactive material and less than 600 mAh/cm³ at the electrode level,respectively. This low capacity of intercalation-type cathodes limitsthe energy density and specific energy of metal-ion batteries.

Fluoride-based cathodes may offer outstanding technological potentialdue to their very high capacities, in some cases exceeding 300 mAh/g(greater than 1200 mAh/cm³ at the electrode level). For example, FeF₃offers a theoretical specific capacity of 712 mAh/g; FeF₂ offers atheoretical specific capacity of 571 mAh/g; MnF₃ offers a theoreticalspecific capacity of 719 mAh/g; CuF₂ offers a theoretical specificcapacity of 528 mAh/g; NiF₂ offers a theoretical specific capacity of554 mAh/g; PbF₂ offers a theoretical specific capacity of 219 mAh/g;BiF₃ offers a theoretical specific capacity of 302 mAh/g; BiF₅ offers atheoretical specific capacity of 441 mAh/g; SnF₂ offers a theoreticalspecific capacity of 342 mAh/g; SnF₄ offers a theoretical specificcapacity of 551 mAh/g; SbF₃ offers a theoretical specific capacity of450 mAh/g; SbF₅ offers a theoretical specific capacity of 618 mAh/g;CdF₂ offers a theoretical specific capacity of 356 mAh/g; and ZnF₂offers a theoretical specific capacity of 519 mAh/g.

In addition, in cases where the fluoride-forming element is inexpensive,fluoride-based cathodes offer low cost potential as well. The 5-yearaveraged wholesale commodity cost of many fluoride-forming elements isreasonably low. For example, in 2013, the cost of Fe was only around$0.2/kg; the cost of Cu was only around $4-9/kg, the cost of Zn was onlyaround $1-2/kg; the cost of Cd was only around $1/kg; the cost of Pb wasonly around $1-2/kg; and the cost of Sb was only around $6-15/kg.

However, many fluorides with high theoretical capacity and hightheoretical energy density (such as CuF₂, NiF₂, PbF₂, BiF₃, BiF₅, SnF₂,SnF₄, SbF₃, CdF₂, ZnF₂, and others) have been believed not to bepractical for use in rechargeable Li-ion batteries due to the previouslyobserved lack of stability and large polarization experimentallyobserved when they were used in conventional cathode configurations,where metal fluorides were mechanically mixed with carbon additives ordeposited on the outer surface of carbon particles.

The advantage of some of these so-called “impractical” fluorides (suchas CuF₂, PbF₂, SnF₂, CdF₂, ZnF₂, and others) over more commonly usedFeF₃ is a more flat discharge curve, which allows the Li-ion batterycell based on such cathodes to maintain a more constant voltage, whileutilizing a large portion of their theoretical capacities.

FIG. 2 compares the discharge curves of CuF₂ with that of example FeF₂and FeF₃-based cathodes, demonstrating significantly flatter performanceof CuF₂.

In contrast to the small structural, chemical, and volumetricdifferences observed during insertion/extraction of Li ions into/out ofcathode intercalation compounds, fluorides exhibit dramatic structuralchanges and significant volume changes accompanying the cell cycling.During Li insertion, a displacement/conversion process takes place,where Li displaces solid fluoride-forming element(s) (such as metals orsemimetals, or in some cases semiconductors), leading to the formationof solid LiF and clusters of the fluoride-forming element(s), typicallyonly 2-10 nm in size. The size of these clusters may be related to themobility of metals in the intermediate reaction products. In cases wherethe diffusion distances between these thermodynamically stablestructures is small and when electrons can be supplied to supportelectrochemical reactions, reversible Li insertion and extraction maybecome feasible.

Theoretically, the Li capacity of fluorides is determined by itsstoichiometry and the density of the fluoride-forming metal according tothe following reaction (which assumes fully reversible transformation):xLi⁺ +xe ⁻+MF_(x) ↔xLiF+M  (Eq. 1)where M is a fluoride-forming element.

Mechanistically, it has been identified that initial insertion of Liinto some of the metal fluorides (such as FeF₂ and FeF₃) takes place asintercalation. For example, during electrochemical reaction of Li withFeF₃, Li first intercalates into the structure forming:Li⁺ +e ⁻+FeF₃→LiFeF₃  (Eq. 2)Only after additional Li insertion, a conversion reaction transforms thereaction products to LiF and interconnected Fe nanoparticles accordingto:2Li⁺+2e ⁻+LiFeF₃→3LiF+Fe  (Eq. 3)

As discussed in the background above, conventional fluoride cathodes maysuffer from limitations, such as (i) low electrical conductivity, (ii)low ionic conductivity, and (iii) volume changes during metal ioninsertion/extraction. Other limitations include (iv) gas generationduring fluoride reactions with electrolytes (particularly at highpotentials), which may cause battery degradation, (v) formation ofsurface species during surface reactions with electrolyte, which mayincrease resistance and reduce reversibility of electrochemicalreactions, (vi) dissolution of the metal fluorides during cycling, whichmay increase resistance, damage the solid electrolyte interphase (SEI)layer on the anode, and reduce both the power performance and cyclestability of battery cells, and (vii) irreversible changes within theirstructure during battery operation, which may also lead to irreversiblecapacity losses.

The present disclosure provides for advanced composite materials forbattery electrodes formed from a nanoporous (preferably with pores inthe range of about 0.5 nm to about 10 nm, but in some designs with poresas large as 500 nm or higher), generally electrically-conductive matrixhaving active fluoride material(s) disposed therein. As discussed inmore detail below, several advantages over conventional designs areprovided by confining active fluoride material into this type of“scaffolding” matrix. For example, confining the active fluoridematerial inside such a matrix (as opposed to surface deposition) helpsavoid the often undesirable agglomeration of individual active materialparticles. As described in more detail below, this matrix mayadditionally help to stabilize particle size, such that active materialexpansion may be accommodated by pores present in a metalfluoride-filled conductive matrix composite. Conductivity properties ofthe matrix may allow reduced resistance and increased rate performance.Scaffolding properties of the matrix may further allow formation ofprotective shells around the composite particles that are mechanicallyand electrochemically stable during cycling and the resulting volumechanges within a metal-fluoride (MF) active material.

FIGS. 3A-3B illustrate two example battery electrode compositionscomprising a fluoride infiltrated into a conductive, nanoporousscaffolding matrix according to certain example embodiments. In each ofthe illustrated example designs, a composite particle is shown withpores 302, conductive pore walls 304, and fluoride 306 infiltratedwithin the pores 302.

The “active” fluoride stores and releases metal ions (e.g., Li ions in aLi battery or a Li-ion battery) during battery operation according tothe reaction described above (Eq. 1). As discussed above, storing andreleasing of these metal ions causes a substantial change in volume ofthe active material, which, in conventional designs, may lead to theloss of electrical contact within the electrode and thus result in arapid loss of capacity during battery operation. In addition, in somecases clusters of the fluoride forming element (e.g., clusters of Cu inthe case of CuF₂) may form isolated particles not electrically connectedto each other. Similarly, the highly electrically insulating LiF maybecome electrically disconnected from the current collector. In thiscase, the conventional design of electrode or electrode particles doesnot allow one to reversibly store and release Li ions during batteryoperation because such processes require uninterruptible transport ofelectrons into (or out of) the active material. Moreover, in some cases(e.g., in the case of CuF₂), the rapid diffusion of a fluoride-formingelement combined, for example, with a high interfacial energy betweenLiF and the fluoride-forming element leads to a physical separation ofthe LiF and fluoride-forming element during Li insertion. Inconventional designs, this greatly limits both the rate andreversibility of the re-formation of an initial fluoride during Liextraction due to mass transport limitations originating from theseparation discussed above.

In the designs shown here in FIGS. 3A-3B, however, the volume changesaccompanying the conversion reactions (such as given by Eq. 1) can beaccommodated by the pores of the nanoporous scaffolding matrix. In orderto maximize the volumetric capacity of the composite material, it may bebeneficial to optimize the pore volume of the fluoride-filledscaffolding matrix in such a way as to have only a small volume of pores(preferably less than about 15%) left unoccupied by LiF and a fluorideforming material after Li insertion to a full discharge. The designsshown here additionally greatly enhance the rate and reversibility ofbattery operation. When the pore size of the scaffolding matrix becomessmall, electrons not only can transport through the matrix material butalso tunnel from the pore walls to the electrochemical reaction sitesduring Li insertion and back during Li extraction. In order to achieve areasonably high reaction rate, it may be advantageous to limit atunneling distance to about 3-5 nm or less. Therefore, the pores of thenanoporous scaffolding matrix may preferably be smaller than 10 nm inwidth.

For designs in which the pores comprise conductive nanoparticles (eitherinitially added or formed during conversion reactions, or, moregenerally, during battery operation), pores in the range of about 10 nmto about 500 nm may be suitable as well. However, in some cases, suchlarger pores may lead to reduced power performance characteristics. Toosmall of pores (e.g., pores less than about 0.5 nm), on the other hand,may make it more challenging to uniformly infiltrate fluorides intotheir pore structure.

Finally, this design confines both the initial fluoride as well asclusters of LiF and fluoride-forming element(s) within the smallnanopores. In this case, the scaffolding matrix forces better contactbetween the clusters of LiF and fluoride-forming element(s), greatlyimproving the rate of charging. Furthermore, if the scaffolding matrixpossesses some elasticity and reasonably high elastic modulus (e.g.,Young's modulus greater than about 5 MPa), then the matrix materialelastically deformed during Li insertion will assist in Li extraction bylowering the reaction energy barrier by its strain energy released bysuch extraction. This additionally enhances the charge rate of abattery.

As one example, porous carbon (e.g., having over 90% sp²-bonded atoms)can serve as an electrically conductive scaffolding matrix. Since thecapacity of carbon in the cathode voltage range of interest (above 1.5 Vvs. Li/Li+ in the case of a Li battery or a Li-ion battery) is small, itmay be preferable to minimize its absolute mass and the volume occupiedby its pore walls, while maintaining the desired pore size andelectrical conductivity. Porous carbons having most of the pore wallsbeing a monolayer-thick have been found to work particularly well,providing both high pore volume and sufficiently high conductivity,while minimizing the volume, which “inactive” carbon atoms occupy. A“perfect” porous carbon particle having slit shaped pores and pore wallscomposed of single-layer graphene may exhibit a specific surface area ofaround 2630 m²/g. Porous carbons with experimentally measured Brunauer,Emmett and Teller (BET) specific surface area above 500 m²/g (morepreferably above 1000 m²/g) have been found to work well as a conductivematrix for fluoride infiltration. An example of a high surface areaporous carbon is activated carbon. It may be produced by pyrolysis ofcarbon-containing organic precursors, followed by its either chemical orphysical activation (partial oxidation of the resulting carbon with thepurpose to enhance its pore volume and specific surface area). It may befurther preferred for porous carbon to have pores occupying at least 50%of the total volume (preferably over 70% and less than around 95% of thevolume). Smaller pore volume provides less space for fluorideinfiltration, and may thus reduce volumetric capacity of thefluoride-infiltrated particles. Conversely, a pore volume above 95%provides reduced mechanical properties to the particles (which maytranslate into poor electrode stability) and additionally makes themmore difficult to handle due to very low density. Conventionallyproduced activated carbon particles often exhibit a majority of pores inthe range of 0.3 nm to 10 nm. Those that have a majority of pores(preferably more than 80% by volume) in the range of about 0.5 nm toabout 10 nm have been found to be particularly useful as a conductivescaffolding matrix for fluoride infiltration.

In some examples, the carbon scaffolding matrix may be doped orfunctionalized with other species (e.g., nitrogen) to increase Li iontransport or to increase its wetting properties.

As is further illustrated in FIGS. 3A-3B, the pores may be straight, asshown in FIG. 3A, or curved, as shown in FIG. 3B. Activated carbonstypically exhibit irregularly-shaped curved pores. The orientation ofthe pores may vary depending on the sample preparation. It may also varywithin a single particle. For example, in the center of the particle,the pores may be oriented randomly or along its radius, while closer tothe surface the pores may be oriented parallel to the surface of theparticle. In this case, advantageous mechanical properties may beachieved. Further, in some cases, the formation of protective coatings,which serve to stabilize metal fluorides against irreversible changesand undesirable interactions with electrolyte (as discussed in moredetail below), may be simplified.

Various methods may be used for fluoride synthesis and infiltration intoa conductive, nanoporous scaffolding matrix. Most of the fluorides canbe synthesized by relatively simple reactions. A first group of suchreactions involves a reaction between an oxide (e.g., metal oxide) and ahydrofluoric acid (HF) either in a gaseous or liquid (solution in water)form. The main feature of this process is that the metal does not changeits oxidation state. Typically, this process may be used to obtainfluorides of the metals in their lower oxidation state (e.g., Sn′, butnot Sn′). Sometimes, in the case of water-soluble fluorides, HF can bereacted directly with the metal (e.g., Zn) to form the metal fluoride(e.g., ZnF₂) solution. Such a solution can be infiltrated into the poresof a matrix and dried to form fluorides confined within the matrix poresof a scaffold.

A second route to obtain metal fluorides is a reaction between metal ormetal oxide with fluorine or fluorinating agents. Examples of the latterinclude, but are not limited to, diethylaminosulfur trifluoride,tris(dimethylamino)sulfonium difluorotrimethylsilicate, and Xefluorides. Because of the strong oxidizing nature of fluorine, theoxidative state of the metal in the resulting fluoride is usually thehighest (2Sb³⁺+5F₂→SbF₅). Another very general route to synthesize waterinsoluble fluorides within the pores of a conductive matrix is an ionexchange reaction between a soluble salt of a fluoride-forming element(e.g., a transition metal) with water-soluble fluoride (typicallyfluoride of alkali metals). Because of the non-solubility of thetransition metal fluoride, the latter precipitates during reaction. Theoxidation state of the transition metal does not change during the ionexchange reaction. The advantage of this method is the exclusion ofhighly toxic and corrosive F₂ and HF from the process. Also, by varyingconcentration of the reagents, order of their mixing, and addition ofsurfactants, the size of the obtained fluoride particles can be tunedover a wide range.

Yet another method for fluoride infiltration into scaffold poresinvolves the use of a soluble fluoride precursor. Such a method mayinclude the following steps: (i) preparation of a solution of a fluorideprecursor, (ii) infiltrating such a solution into the scaffold pores,(iii) subsequent solvent evaporation (steps ii and iii may be repeatedmultiple times to achieve high fluoride loadings), and (iv) thermalannealing to decompose a fluoride precursor into a fluoride and avolatile specie, which can be removed. For example, an aqueous solutionof a salt of a fluorosilicic acid (e.g., FeSiF₆.H₂O) can be impregnatedinto the scaffold pores, dried, and the produced salt can be transformedinto a metal fluoride (e.g., an iron fluoride FeF₂) by annealing (forexamples, in an inert argon (Ar) gas). The annealing process decomposesFeSiF₆.H₂O into FeF₂ (more generally FeF_(x)), water vapors, andvolatile SiF₄ (SiF₄ boiling point=86° C.), with volatile compounds beingremoved. In some cases, initial annealing in an oxygen-containing gasmay induce formation of the FeF₃ from the same precursor. The process ofmetal fluoride (MF) infiltration may be repeated multiple times toachieve the desired fraction of MF in the composite.

As mentioned above, for soluble salts the simplest method of theirinfiltration into the porous scaffold may involve the following steps:(i) wetting the scaffold with a salt solution in solvent with (ii)subsequent solvent evaporation. This procedure can also be repeated oneor more times in order to increase loading of the scaffold with thefluoride material. Solutions of different fluorides can be combined oralternatively applied in order to obtain mixed fluoride compositionswithin the scaffold. Solvent-insoluble fluorides can be prepared in theform of a suspension of small nanoparticles in suitable liquid media.Then, the porous scaffold may be soaked with a nanoparticle suspensionand the liquid evaporated. In order to increase metal fluoride loading,the procedure may be repeated. The size of such particles shouldgenerally be smaller than the size of the pores. In some cases,impregnation with solution and suspension can be combined for thepurpose of filling the scaffold with a mixture of different compounds.This might be necessary for the enhancement of material capacity orimprovement of rate performance.

In yet another approach, a fluoride can be formed directly within thescaffold pores, using a reaction between a fluoride precursor (such asmetal oxide, metal hydroxide, or metal nanoparticles) with HF, F₂, or asolution of a fluorinating agent. The precursors can be introduced intothe scaffold by solution impregnation, soaking of the scaffold withprecursor suspension, vapor infiltration (capillary condensation) ofvolatile Me compounds, and other routes. After impregnation with afluoride precursor, the scaffold can be subjected to HF vapor, F₂, or HFsolution in the case of forming of insoluble fluorides. In many cases,the fluorinating agent can be soluble in organic solvents, such that theformed fluoride remains within the scaffold pores.

In the case of relatively volatile metal fluorides (e.g., SbF₅, SbF₃,BiF₅, SnF₂, and others), scaffold filling with the active material canbe performed by capillary condensation of the fluoride vapor. Because ofcapillary action, vapor is preferentially condensed into scaffoldcapillaries, starting from the smallest pores, which exhibit thestrongest interactions with the vapors. Because a typical desiredscaffold pore size may be on the order of nanometers (e.g., about 1 nmto about 10 nm), it is possible to obtain conditions for very selectivein-pore condensation, avoiding any condensation on the outside surfaceof the scaffold or in larger pores.

In yet another approach, fluoride nanoparticles (including agglomeratednanoparticles forming porous bodies) may be first produced and theninfiltrated with a precursor for the scaffolding matrix material. Insome examples, a polymer or a blend of polymers (or co-polymers) may beused as a precursor for the scaffolding matrix. Upon thermal treatment,the precursor may transform into the porous conductive scaffoldingmatrix. An outer protective coating may further enclose the producedcomposite fluoride-comprising particles, as described further in moredetail for various types of the fluoride and scaffolding matrixcomprising particles.

Depending on the particular application, the scaffold material can befilled with various active materials. Combinations of theabove-described methods accordingly provide the ability to achieve awide range of desired composition and morphology of the active particleswithin the scaffold.

The porosity of the scaffolding matrix need not be uniform. Moreover, insome example designs, the relative changes in the average pore fractionof the conductive matrix from the center to the surface of the particlesmay provide unique advantages. For example, having a larger pore volumein the center (or the bulk) of the particle may allow higher MF loadingand higher energy density. At the same time, having a smaller porevolume near the surface of the particle may provide improved mechanicalstability of the composite particle. It may also facilitate theformation of more conformal and more stable protective coatings. Such acoating may infiltrate into at least some of the pores of theMF-infiltrated scaffolding matrix.

FIGS. 4A-4B are graphical depictions illustrating four particularexamples of a porous scaffolding matrix with different relative changesin the average pore fraction of the matrix from the center to thesurface of the particles (different profiles of the pore volume). In thedesign of FIG. 4A, the bulk of the pore volume is more or less uniformin the bulk of the particle, but is substantially reduced near thesurface layer. In the design of FIG. 4B, the porosity of the matrixmaterial gradually decreases from the center to the surface of theparticle. In the design of FIG. 4C, the top surface layer may comprise adifferent morphology and composition. Two or more layers may be presentnear the surface, to achieve improved performance, where thenear-surface layer(s) may comprise a larger pore volume than the topsurface layer. In the design of FIG. 4D, the near-surface layer maycomprise a smaller pore volume than the top surface layer. In this case,a protective coating may first seal the MF-infiltrated matrix particlein the near-surface layer, while the larger pores of the top surfacelayer may prevent sintering of the particles during coating formation.

FIG. 5 illustrates a schematic of charge and discharge processes withinan example (single) pore, where Li ions react with a fluoride confinedwithin the pore, forming nanoclusters composed of a fluoride-formingelement and nanoclusters of LiF. In the illustrated example, thecomposite material cycles between (i) a first state comprising a pore502 (e.g., less than about 6 nm) and fluoride (MFx) 504 within porewalls 506 and (ii) a second state comprising clusters of afluoride-forming element(s) (M) 508 interspersed with LiF 510 within thepore walls 506.

Even when the produced fluoride forming elements (e.g., nanoclusters ofCu in the case of CuF₂) are not connected to each other, they remainwithin the electron tunneling proximity of conductive pore walls. As aresult, the electrons needed for the reactions of Eq. 1 can be providedby the conductive matrix. In addition, the confinement of the reactionspecies within the pore walls enhances the contact between a cluster ofF element(s) (e.g., nanoclusters of Cu in the case of CuF₂) and LiF,which improves the rate of electrochemical reactions (as evidenced byEq. 1). Finally, the interactions with the pore walls may reduce thedissolution of the cathode active material into the electrolyte.

Formation of chemical bonds between fluoride cluster(s) and the porewalls may be beneficial in some designs. For example, such bonds limitthe movement of clusters of LiF and fluoride forming element(s) withinthe pores during battery operation, improving the long-term stability ofthe fluoride porous matrix composite. Such bonds may form duringfluoride infiltration into the functional groups' decorated porousmatrix and may involve other species, such as oxygen or sulfur atoms.

In some applications, reaction(s) of either fluorides orfluoride-forming elements (such as metals) with the electrolyte maycause undesirable side reactions, such as electrolyte oxidation,formation of gases, or dissolution of the fluoride-forming elements intothe electrolyte as a result of chemical or electrochemical etching. Theelectrolyte decomposition may form a film, which may prevent or slowdown reversible transformations during battery operation. Formation ofgaseous species may result in undesirable cell expansion andadditionally reduce the rate capability of the cell. Dissolution oretching of active material elements reduces the cathode capacity. Inorder to overcome such problems, the protective layer discussed abovemay conformally encase the fluoride-infiltrated porous matrix core insuch a way as to avoid direct contact between the electrolyte solventspecies and fluoride as well as fluoride-forming elements. The surfacediffusion of Li ions is typically faster than their diffusion within thebulk of the material. In some cases, even though the pores are free fromelectrolyte, the diffusion of Li ions into/out of the fluoride porousmatrix composite is sufficiently fast to allow for battery operation atsufficiently fast rates (for a given application). In this case, theouter surface area of the scaffolding matrix may be used for thedeposition of an ionically conductive (and solvent impermeable) outershell, thereby sealing the active material deposited inside thescaffolding matrix and avoiding the often undesirable contact of activematerial with solvent molecules of the electrolyte.

FIGS. 6A-6B illustrate two particular examples of fluoride-containingcomposites, where active fluoride nanoparticles or active fluoridenanoplatelets are confined within a conductive matrix of a nanoporousmaterial, and where the overall fluoride matrix composite particle isfurther coated with a protective layer of electrolytesolvent-impermeable shell material conductive to Li ions. In each of theillustrated example designs, a composite particle comprises pores 602(e.g., less than about 10 nm), conductive pore walls 604 of the particlecore, fluoride 606 infiltrated within the pores 602 of the particlecore, and a respective shell, such as an “active” shell 610 that iselectrolyte solvent impermeable and composed of another active (Li-ionstoring) but intercalation-type material, which serves several purposes,or an “inactive” shell 612 that is electrolyte solvent impermeable (butpermeable to Li ions) and protects the fluorides from undesirablereactions with electrolyte.

In some examples, a protective shell material may comprise one of thefollowing types of materials: an oxide, an oxyfluoride, a sulfide, afluoro-sulfide, an oxysulfide, a fluoride, a non-fluoride halide, anitride, an oxynitride, a hydride, a polymer, or a carbon

In more detail, FIG. 6A illustrates an example of fluoride-containingcomposites having an “active” material shell also capable of storing Liions during discharge of a Li battery or a Li-ion battery. FIG. 6Billustrates an example of fluoride-containing composites having alargely “inactive” material shell not capable of storing more than, forexample, 50 mAh/g of Li within the voltage range to which thefluoride-matrix composite particles are exposed during batteryoperation. In some applications, it may be advantageous for the shellmaterial to exhibit relatively small volume changes during insertion andextraction of Li ions. Therefore, in cases when a protective shell is an“active” material, it may be advantageous for this material to store Liions by intercalation within the potential range of cathode operation,as intercalation reactions generally involve relatively small volumechanges.

In some cases, it may be advantageous for the protective shell to be“multi-functional” by: (i) having the ability to transport Li ions andat the same time prevent transport of solvent molecules to thefluoride-containing core, (ii) having an additional function to store Liions (to serve as an active material) within the operating potentialrange (as previously illustrated by FIG. 6A), (iii) providing additionalmechanical strengthening functionality (to help minimize volume changeswithin the particle during Li-ion insertion), and (iv) enhancingelectrical conductivity (to contain an electrically conductive outerlayer, such as a conductive polymer layer, conductive carbon layer,conductive metal layer, or conductive oxide layer).

In some applications, it may be advantageous for the protective shell tobe a composite. The shell may be a composite material comprising atleast an inner layer and an outer layer, with potentially one or moreother layers therebetween. The shell may accordingly be made bycombining different coatings and different layers may be provided fordifferent functions. For example, one component of the shell may providebetter structural strength, and another one better electronicconductivity. In another example, one component can provide better ionicconductivity, and another one better electrical conductivity. In someapplications, it may be advantageous to have these componentsinterpenetrate each other. In this case, the composite shell may provideboth high ionic and electrical conductivity if one component is moreelectrically conductive and another one more ionically conductive.

Further, a portion of the porous scaffolding matrix of the core can beleft exposed and, therefore, be used for the stable attachment of a(polymer) binder. A polymer binder may also be firmly attached to aprotective shell. A more stable particle-binder interface may lead tomore stable performance of the electrode.

FIGS. 7A-7B illustrates two particular examples of fluoride-containingcomposites with a protective shell layer being a composite materialaccording to various embodiments. As in the example designs of FIG. 6 ,each of the illustrated example designs here includes a compositeparticle that comprises pores 602 (e.g., less than about 10 nm),conductive pore walls 604 of the particle core, and fluoride 606infiltrated within the pores 602 of the particle core. In addition, eachof the illustrated example designs here includes a respective compositeshell, such as an “interpenetrating” composite shell 710 that iselectrolyte solvent impermeable and based on a composite ofintercalation-type material impregnated within the porous scaffold ofelectrically conductive material, or a combination of an outerprotective shell layer 712 that is composed, for example, of anintercalation-type material, and an inner protective shell layer 714that is a composite of fluoride impregnated within a porous scaffold ofelectrically conductive material.

In more detail, FIG. 7A illustrates a conformal shell composed of anintercalation-type active material (for example, a lithium metal oxideor a lithium metal phosphate) interpenetrating a porous electricallyconductive material (for example, carbon). FIG. 7B illustrates aconformal shell composed of two layers: (i) an outer layer ofintercalation-type active material and (ii) an inner layer of a metalfluoride infiltrated into an electrically conductive porous shell. Oneadvantage of having a fluoride-filled electrically conductive porousshell is that if the pore size in such a shell is smaller than that ofthe porous conductive matrix core, it becomes easier to “seal it”(making it impermeable to solvent molecules) with a Li-ion conductivematerial, without impregnating significant amounts of such material intothe core.

In some applications (for example, when the particle size is large orwhen the Li ion transport with the electrolyte-free porousfluoride-containing core is slow) it is advantageous to have so-called“channel pores” (e.g., greater than about 6 nm) within thefluoride-containing composite particles, which provide more rapidtransport of ions from the outside of the particles to their core.

FIG. 8 illustrates two particular examples of fluoride-containingcomposites according to various embodiments, where a fluoride-conductivecore contains both relatively small (e.g., smaller than 6 nm) pores toconfine fluorides and relatively large “channel” pores (e.g., greaterthan 6 nm) designed to provide rapid transport of ions from theelectrolyte solution into the core of the particles. As in the exampledesigns of FIG. 6 , each of the illustrated example designs hereincludes a composite particle that comprises pores 602 (e.g., less thanabout 6 nm), conductive pore walls 604 of the particle core, andfluoride 606 infiltrated within the pores 602 of the particle core, withor without a protective shell 610. In addition, each of the illustratedexample designs here includes channel pores (e.g., greater than about 6nm) for fast Li ion access to the core.

In more detail, in FIG. 8A these channel pores are not filled withactive material or a solid electrolyte so that the ion transport in suchpores proceeds in a liquid phase (that is, via liquid electrolytefilling the pores). In FIG. 8B these channel pores are filled eitherwith an active material having high ionic conductivity or a solidelectrolyte so that the ion transport in such pores proceeds via a solidphase. In this case, the filled channel pores may provide an additionalmechanical strengthening function, further assisting particles tomaintain constant size and avoid fracture during Li ion insertion andextraction.

In some applications, it may be advantageous to keep the dimensions ofthe battery (e.g., a thickness of a pouch cell) as constant as possibleduring battery operation. However, high capacity electrodes, such asanode particles based on a Li alloy-type active material or cathodeparticles based on conversion-type active material, may exhibit somevolume changes during Li shuttling between the anode and cathode. Someof the volume changes may be minimized by using some of the approachesdescribed herein, in accordance with a porous scaffolding matrix thatexhibits relatively little volume changes during Li ioninsertion/extraction. However, some of the volume changes may remain inat least one of the electrodes. Such thickness and volume changes may beintroduced even intentionally in order to maximize the electrode'svolumetric energy storage characteristics. In order to further preventchanges in thickness at the cell level, the expansion of both electrodesmay be tuned in such a way as to compensate for the total volume changesand bring them to an acceptable level (e.g., to below 3% in someapplications).

In some designs, it may be advantageous to introduce a gradual change inthe concentration of the protective material from the center to thesurface of the composite particles. In this case, the overall stabilityof the particles may be enhanced. The lack of abrupt compositionalchanges makes the particles more resilient against fractures duringhandling and electrode preparation. In addition, even with the formationof surface cracks, the protection may still function efficiently for atleast a portion of the individual particle. A higher fraction of theprotective material (e.g., intercalation-type active material) near thesurface of the particle may help to improve particle stability, while alower fraction of the active material in the center of the particles mayhelp to improve particle energy density.

FIGS. 9A-9D illustrate four particular examples of 1W-filled conductivematrix particles additionally comprising protective material, such asintercalation-type active material or simply a Li-ion conductivematerial (which may exhibit low capacity, e.g., lower than about 50mAh/g in the potential range of particle operation in a battery cell)that prevent undesirable reactions with electrolyte and improve batterycell stability and performance. As in the example designs of FIG. 6 ,each of the illustrated example designs here includes a compositeparticle that comprises pores 602 (e.g., less than about 6 nm),conductive pore walls 604 of the particle core, and fluoride 606infiltrated within the pores 602 of the particle core, with or without aprotective shell 910, composite shell 912, additional coatings 914, andso on. In addition, each of the illustrated example designs hereincludes a gradient filling 930 of intercalation-type material that mayprotect the MF from undesirable interactions with electrolyte. In theseexamples the volume fraction of the MF may be either uniform within thescaffolding matrix or vary according to a specific profile (e.g., havingsmaller volume fraction of the MF near the surface of the particles).

In more detail, FIG. 9A illustrates a simplest case of MF-infiltratedscaffolding matrix particle gradually filled with the protective coatingmaterial. FIG. 9B illustrates an example of a similar material having anadditional layer of a thicker protective coating material on the outersurface of the particles. Such a layer may preferably contain as smallfraction of the MF as feasible. However, some content of MF might beunavoidable during synthesis. A suitable thickness of the surface layermay range, for example, from about 2 nm to about 200 nm. The volumefraction of the intercalation type material may be a compromise, as ahigher fraction may lead to better stability and higher capacity, whilea smaller fraction may result in a higher energy density of thecomposite particles. FIG. 9C illustrates a similar particle, but with anouter layer being a composite. In some examples, as previouslydescribed, such a composite may comprise two or more interpenetratingcomponents (e.g., an electrically conductive component and a Li-ionstoring component).

FIG. 9D illustrates another example, where the outer layer of theprotective shell material may be additionally covered with a functionalshell layer. Such a layer may serve different functions. For example, itmay increase electrical conductivity of the material, improveinteractions with the electrolyte, improve dispersion in a slurry,improve adhesion to the binder, or simply further improve mechanicalstability of the particles. This layer should be permeable to the activematerial ions of interest (such as Li ions in the case of Li or Li-ionbatteries). It may be continuous and conformal or discontinuous (e.g.,composed of nanoparticles). Conductive polymers (particularly those thatremain electrically conductive within the potential range of theelectrode operation), conductive carbon (such as carbon comprisingmostly sp² bonded C atoms), metals (e.g., transition metals, such as Fe,Al, and others, or, e.g., rare earth metals or their salts that largelydo not dissolve in electrolyte), semimetals (e.g., Si, Sn, and others),metal sulfides (e.g., titanium sulfide, iron sulfide, and others), metalfluorides (particularly those that do not experience conversionreactions in the potential range of electrode operation and have lowsolubility in electrolytes), metal oxides, and mixtures of theabove-discussed materials are examples of suitable compositions of sucha functional layer.

In some designs, the protective coating may be formed directly on thesurface of the metal fluoride (MF) active material dispersed within theconductive scaffolding matrix particles.

FIGS. 10A-10B illustrate two examples of a portion of a composite whereMF is confined within the conductive material pore (e.g., a carbon pore)and additionally coated with a protective material coating layer 1010.As shown, the protective material coating layer 1010 may be formed as acoating on the MF surface (as in FIG. 10A) or as a coating of a secondMF2 material on a first MF1 material surface (as in FIG. 10B). Thesuitable thickness of such a layer may depend on the size of the poreand the size of the MF particles. As an example, suitable thickness maytypically range from about 0.2 to about 20 nm.

The volume fraction of the MF within the scaffolding matrix need not tobe uniform. A reduced volume fraction of the MF near the surface of theconductive matrix particles may improve the mechanical andelectrochemical stability of the particles when used in metal-ionbatteries.

FIGS. 11A-11B illustrate two particular examples of the changes in theaverage volume fraction that may be occupied by the MF along theparticle radius from the center to the surface of the particles incertain embodiments.

In some designs, the mass or volume fractions of the protective materialdistribution within the composite particles may also change from thecenter to the surface of the composite.

FIGS. 12A-12D illustrate examples of changes in the average massfraction of the protective material along the radius of the compositeparticles. FIGS. 12A and 12B illustrate a growing fraction of theprotective material near the surface and a smaller fraction of theprotective material in the core of the particles. FIG. 12C illustratesan example where the protective material virtually disappears near thecenter of the particles and is mostly limited to the surface region.FIG. 12D illustrates an example where the protective material isuniformly distributed within a composite (e.g., when it is directlyapplied to the surface of the MF particles confined within the pores ofthe conductive matrix).

As discussed above, an intercalation-type active material may beincorporated into the MF-comprising composite particles. In someexamples, such incorporation serves to improve stability of the MF(e.g., functioning as a protective layer). In other examples, suchincorporation serves to provide high current in pulse regimes.Intercalation-type materials typically exhibit higher rate performanceand thus their incorporation may allow high current pulses of thebattery to be supported by the composite electrodes. Examples ofsuitable intercalation-type active materials are noted hereinabove.These intercalation compounds are often divided into several crystalstructures, such as layered, spinel, olivine, and tavorite, to name themost common ones. The layered structure is the earliest form ofintercalation compounds for the cathode materials in Li-ion batteriesand metal chalcogenides, including TiS₃ and NbSe₃, and have been studiedpreviously as a possible intercalating cathode material. They do notserve as high energy density materials but may provide adequate powerperformance and protection for MF. Layered LiCoO₂, LiMnO₂,LiNi_(x)Mn_(y)Co_(z)O₂, and LiNi_(x)Co_(y)Al_(z)O₂ (where x, y, and zrange from 0 to 1) are more common in devices. Typical examples of aspinel include LiCo₂O₄ and LiMn₂O₄. Typical examples of olivine activematerials include LiFePO₄, LiCoPO₄, and LiMnPO₄. Olivines belong to abroader class of polyanion compounds. Large polyanions (XO4)3-(X=S, P,Si, As, Mo, W) occupy the lattice and these anions, increase cathoderedox potential, and stabilize its structure. Non-olivine polyanionintercalation-type active materials may also be used. Tavorites is acommon example of none-olivine polyanion intercalation type activematerials. Typical examples of tavorite materials include LiFeSO₄F andLiVPO₄F.

As can be seen from the examples above, the majority of active materialsmay contain up to approximately 50 atomic % oxygen. Some of the suitableactive materials also contain transition metal atoms, including Fe. Dueto the low cost of Fe, the use of such an element in the protectivematerial may provide cost advantages.

When designing composite materials comprising both MF andintercalation-type active materials, it may be advantageous to make surethe intercalation-type active material is electrochemically stable inthe potential range of the electrode operation. In some examples, it maybe advantageous for the active material to exhibit the highest capacityin the potential range of cathode operation (typically from above 1V vs.Li/Li+ to below 4-4.2 V vs. Li/Li+).

FIGS. 13A-13D illustrate four particular examples of MF-filledconductive matrix particles additionally comprising intercalation-typeactive material. In these examples, the fraction of theintercalation-type active material changes from the center to theperimeter (surface) of the composite particles.

The use of two or more types of metal fluorides within the compositeparticles may be advantageous in some applications. Some metal fluoridesexperience reduced dissolution in contact with electrolyte (at leastwhen used in the potential range of cathode operation), more favorableinteraction with electrolyte (e.g., forming a favorable and stable solidelectrolyte interphase layer), form an interconnected network of metalnanoparticles during the conversion reaction (such that rate performancemay be higher), or provide other attractive properties. At the sametime, these metal fluorides may suffer from higher cost or lower energydensity than other fluorides (for example, lower than that of CuF₂).Combining different types of metal fluorides into one composite mayprovide a combination of attractive properties, such as high energydensity, high rate, and high stability performance.

FIGS. 14A-14C illustrate three particular examples of a compositeparticle comprising a scaffolding matrix and two types of metalfluorides (MF1 and MF2). In FIG. 14A, the ratio of the mass fractions ofthe metal fluorides is uniform within the particle (no changes from thecenter to the perimeter of the particle). In FIGS. 14B and C, a highercontent of MF2 is present near the surface of the composite particle.

FIG. 15 illustrates an example of a building block of a Li-ion batterywith volume and thickness changing electrodes (e.g., changes of over 6vol. % in each) carefully designed to counter-balance the changes ofeach other and minimize the changes in the battery's dimensions duringits operation. In this example, the cathode cycles between an expandedstate 1502 and a compacted state 1504, while the anode cycles between acompacted state 1506 and an expanded state 1508. For example, if theanode exhibits a 10% increase in thickness during Li-ion insertion, theporosity and architecture of the cathode particles (shown, for example,in the porous particles of FIGS. 2-14 ) can be tuned such that thecathode shrinks by 7-13% during Li extraction. In this way, the overallvolume changes remain below 3%.

In some applications, the use of a solid electrolyte or a solidelectrolyte layer may provide benefits of improved stability.

FIG. 16 illustrates a particular example of an electrode comprisingfluoride infiltrated into a conductive scaffolding matrix of ananoporous material, where the electrode is either further coated with alayer of a solid electrolyte or a solid electrolyte completelyinfiltrates the pores between the individual fluoride-containingcomposite particles. As shown, the electrode may comprise a currentcollector 1602, composite fluoride-containing active particles 1604exhibiting relatively very small (e.g., less than about 15%) volumechanges during insertion and extraction of Li ions, and a solidelectrolyte layer 1606 coating the electrode particles. Because thescaffolding matrix reduces the volume changes within the compositeparticles (compared to that of the plain MF), these composite particlesmay offer enhanced stability when combined with a solid electrolyte inbattery cells.

In many of the figures discussed above, the shape of the particles isshown as near-spherical. In some applications, the use of irregularlyshaped particles may offer reduced powder fabrication costs.

FIG. 17 illustrates a particular example of an irregularly shaped activeparticle comprising a fluoride infiltrated in a conductive nanoporousmatrix. As in the example designs of FIG. 6 , the illustrated exampledesign here includes a composite particle that comprises pores 1702(e.g., less than about 10 nm), conductive pore walls 1704 of theparticle core, and fluoride 1706 infiltrated within the pores 1702 ofthe particle core. In other applications, the use of cylindrical,fiber-shaped, or platelet-shaped particles may offer certain benefits(such as higher electrical interconnectivity within the electrode orhigher packing density).

In some designs, it may be advantageous to use porous scaffolding matrixparticles with pores larger than 10 nm. Larger pores may be easier touniformly fill with fluorides.

FIGS. 18A-18D illustrate four particular examples where utilizingscaffold particles with large pores may still be attractive. Pores aslarge as 500 nm, for example, may work in some designs. However, theirperformance may be a trade-off. In the illustrated example designs, eachpore includes fluoride 1802 infiltrated within electrically conductivepore walls 1804, and variously includes an electrically conductivecoating 1806, electrically conductive particles 1808, and/orelectrically conductive segments 1810, which may encase fluorideclusters (nanoparticles).

In more detail, FIG. 18A illustrates a large pore with clusters of metalfluorides coated with thin (preferably less than about 1 nm) layers ofelectrically conductive material (such as carbon) in order to providepaths for electron transport to electrically insulative fluorides,including LiF, needed for reversibility of electrochemical reactions(see Eq. 1).

FIG. 18B illustrates an example where fluoride clusters (nanoparticles)contain electrically conductive nanoparticles (of various shape orcomposition). The conductive particles may comprise metals, conductivecarbon, conductive ceramic particles, and others. In this case,electrons can tunnel from one conductive nanoparticle to another inorder to participate in electrochemical reactions. Such particles mayalso serve another function—reducing mobility of the fluoride-formingelement and LiF so that both of these materials remain in closeproximity to each other so that reversible electrochemical reactions canproceed faster. Yet another potential function of such particles is tocatalyze the electrochemical reactions to reduce the potential barrierand, therefore, reduce overpotential, thus increasing the energyefficiency of the battery cell and increasing the operating voltage(thus increasing energy density). In addition, reduction of the energybarrier for these transformations enhances the rate performance andpower characteristics of the batteries.

FIG. 18C illustrates an example where fluoride clusters (nanoparticles)contain both nanoparticles (having at least one of the three functionsdescribed above) and the surface coating, which may either enhanceconductivity or reduce the mobility of the reaction products, or performboth functions.

FIG. 18D illustrates an example where multiple fluoride clusters(nanoparticles) form composites with an electrically conductive material(such as carbon) and such composites are confined within these largerpores. In this case, electrons will be transferred through theconductive portion of such composites (e.g., through carbon if carbon isused for composite formation).

FIG. 19 includes a flow chart illustrating an example method offabricating a metal fluoride-containing composite electrode. In thisexample, the fabrication method includes: (i) providing an activefluoride material to store and release ions during battery operation,whereby the storing and releasing of the ions causes a substantialchange in volume of the active material (block 1902); and (ii) forming ananoporous, electrically-conductive scaffolding matrix within which theactive fluoride material is disposed, wherein the scaffolding matrixstructurally supports the active material, electrically interconnectsthe active material, and accommodates the changes in volume of theactive material (block 1904). In some designs, forming the scaffoldingmatrix may comprise, for example, forming a carbon-containing precursor,oxidizing and carbonizing the carbon-containing precursor to form acarbonized particle, and activating the carbonized particle at elevatedtemperature to form the scaffolding matrix with a pore volume of greaterthan 50 vol. %. In some designs, the active fluoride material-infusedscaffolding matrix may be formed, for example, as a powder comprisingparticles, with the method further comprising mixing the active fluoridematerial-infused scaffolding matrix particles with a binder, and castingthe binder-bonded particles onto a metal foil current collector. In somedesigns, the method may further comprise forming a shell at leastpartially encasing the active fluoride material and the scaffoldingmatrix, the shell being substantially permeable to the ions stored andreleased by the active material. In some designs, at least a portion ofthe shell material may be deposited by chemical vapor deposition. Insome designs, at least an outer portion of the shell may be depositedelectrochemically during one or more initial battery cycles, duringwhich electrochemical decomposition of at least some electrolytecomponents occurs.

For some designs, the protective coating(s) may be deposited from avapor phase via vapor deposition methods. Examples of such methodsinclude, but are not limited to, chemical vapor deposition (CVD), atomiclayer deposition (ALD), plasma-enhanced ALD, plasma-enhanced CVD, vaporinfiltration, and others. For some designs, the protective material maybe deposited from a solution. Examples of suitable methods includesol-gel, layer-by-layer deposition, polymer adsorption, surfaceinitiated polymerization, nanoparticles adsorption, spray drying, andothers.

FIGS. 20A-20B include a flow chart illustrating another example methodof fabricating a metal fluoride-containing composite electrode. In thisexample, the fabrication method includes: (i) providing a porousconductive matrix (e.g., in a powder form) (block 2002); (ii)infiltrating the matrix (e.g., particles) with metal fluoride(s) activematerial (block 2004); (iii) (optionally) depositing intercalation-typeactive material within the matrix (optional block 2006); (iv)(optionally) depositing a protective coating on the accessible surfaceof the metal fluoride(s) (optional block 2008); (v) (optionally)depositing a metal ion (e.g., Li-ion) conductive protective coating onthe surface of the composite particles, forming closed poresinaccessible by electrolyte solvent molecules (optional block 2010);(vi) (optionally) depositing a functional coating on the surface of thecomposite particles (optional block 2012); and (vii) (optionally) mixingthe composite (particles) with a binder and depositing on the currentcollector substrate, thus producing a composite particles-basedelectrode (optional block 2014).

The protective coating(s) may be formed not only before electrodeformation but also after electrode formation (but before batteryassembling). The coating may be deposited by vapor deposition methods(e.g., as described above), by solution deposition methods (e.g., asdescribed above), by electrodeposition, by electroless deposition, andby other known methods of coating deposition.

FIGS. 21A-21B include a flow chart illustrating another example methodof fabricating a metal fluoride-containing composite electrode. In thisexample, the fabrication method includes: (i) providing a porousconductive matrix (e.g., in a powder form) (block 2102); (ii)infiltrating the matrix (e.g., particles) with metal fluoride(s) activematerial (block 2104); (iii) (optionally) depositing anintercalation-type active material within the matrix (optional block2106); (iv) (optionally) depositing a protective coating on theaccessible surface of the metal fluoride(s) (optional block 2108); (v)(optionally) depositing a functional coating on the surface of thecomposite particles (optional block 2110); (vi) (optionally) mixing thecomposite (particles) with a binder and depositing on the currentcollector substrate, thus producing a composite particles-basedelectrode (optional block 2112); and (vii) (optionally) depositing ametal ion (e.g., Li-ion) conductive protective coating on the accessiblesurface of the composite electrode, forming closed pores within thecomposite inaccessible by electrolyte solvent molecules (optional block2114).

The protective coating(s) may also be formed in-situ after cellassembling. In this case, the products of the reduction (or oxidation orboth) of at least one of the component(s) of the electrolyte on thesurface of the electrode particles during the first cycle (at lowtemperature, at room temperature, or at elevated temperature (e.g.,30-80° C.)) may induce formation of a suitable protective layerpermeable to active metal ions (such as Li ions in the case of Li orLi-ion batteries). In some examples, formation of suitable coatings maybe catalyzed by the presence of salt additives in the electrolyte or bythe surface species on the surface of the composite particles. In someexamples, rare earth element-comprising salt additives (e.g.,La-comprising salt additives), halogen element-comprising salt additives(e.g., I- or F-comprising additives), and sulfur-comprising saltadditives may work well for the formation of protective coatings.

FIGS. 22A-22B include a flow chart illustrating another example methodof fabricating a metal fluoride-containing composite electrode with aprotective coating formed in-situ after battery assembling. In thisexample, the fabrication method includes: (i) providing a porousconductive matrix (e.g., in a powder form) (block 2202); (ii)infiltrating the matrix (e.g., particles) with metal fluoride(s) activematerial (block 2204); (iii) (optionally) depositing intercalation-typeactive material within the matrix (optional block 2206); (iv)(optionally) depositing a protective coating on the accessible surfaceof the metal fluoride(s) (optional block 2208); (v) (optionally)depositing a functional coating on the surface of the compositeparticles (optional block 2210); (vi) (optionally) mixing the composite(particles) with a binder and depositing on the current collectorsubstrate, thus producing a composite particles-based electrode(optional block 2212); (vii) providing an electrolyte capable of forminga Li-ion permeable (and largely solvent impermeable or transition metalion impermeable) layer during one or more electrochemical cycles (block2214); and (viii) assembling a battery cell comprising the producedelectrode and suitable electrolyte, and inducing in-situ formation of aprotective layer by changing the electrochemical potential of the metalfluoride electrode (block 2216).

Conventional cells typically operate with 0.7-1.3M electrolytes. Iforganic electrolytes are used in the battery cells with MF-conductivematrix composites, the metal ion (such as Li-ion) suitable saltconcentration may range from about 0.1 M to as high as about 12M.Ultra-high molarity electrolytes (e.g., 3-12M, if such highconcentrations can be achieved in electrolytes without saltprecipitation), in particular, may significantly improve stability ofthese composites (particularly if protective coatings are not present orare imperfect) by significantly reducing dissolution of the metalfluorides during cycling. It may be particularly advantageous to utilizean electrolyte with solvation energy of the Li-ion salts higher thanthat of the metal ions or metal-comprising ions (which may be producedduring the conversion reactions within metal fluorides).

In some designs, one or more of the fluoride-containing compositionsdescribed above may contain Li.

In some designs, it may be advantageous to pre-lithiate metal fluorideelectrodes prior to using them in rechargeable Li or Li-ion batterycells. Several methods have been found suitable. These include but arenot limited to: (i) direct contact with a Li metal or Li metal alloycomposition, (ii) chemical lithiation, and (iii) electrochemicallithiation. For electrochemical lithiation, Li is inserted into thefluoride-comprising electrode from an electrolyte under an applicationof an electrical current. Insertion of Li ions is accompanied either byreplenishing Li cations into electrolyte from another Li-containingelectrode (e.g., a Li foil) or by formation of reduced species (e.g., ina gaseous, a liquid, or a solid form) at the counter electrode (from thecounter anions).

The process of electrochemical lithiation may also take place within apre-assembled or a fully assembled cell. In this case, a third electrode(in addition to anodes and cathodes) may be used as a Li source. Thisthird electrode may preferably have a high Li content. In one example, aLi foil (e.g., surface protected Li foil) may serve as this sacrificialelectrode for supplying Li ions for lithiation. In some applications,such a third electrode may be (nearly) completely consumed.

The description is provided to enable any person skilled in the art tomake or use embodiments of the present invention. It will beappreciated, however, that the present invention is not limited to theparticular formulations, process steps, and materials disclosed herein,as various modifications to these embodiments will be readily apparentto those skilled in the art. That is, the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention.

The invention claimed is:
 1. A composite particle, comprising: a mixtureof metal and lithium fluoride (LiF) materials; a skeleton matrixmaterial into which the mixture is embedded to form an active materialcore; and a conformal Li-ion permeable shell at least partially encasingthe active material core.
 2. The composite particle of claim 1, whereinthe skeleton matrix material comprises pores having an averagecharacteristic pore width in a range of about 1 nm to about 10 nm. 3.The composite particle of claim 1, wherein a shape of the compositeparticle is substantially spherical.
 4. The composite particle of claim1, wherein the conformal Li-ion permeable shell has an average shellthickness in the range of about 0.2 nm to about 100 nm.
 5. The compositeparticle of claim 1, wherein a volume fraction of the skeleton matrixmaterial is different near the perimeter of the composite particle thanin the center of the composite particle.
 6. The composite particle ofclaim 5, wherein the volume fraction of the skeleton matrix materialnear the perimeter of the composite particle is at least 10% larger thanin the center of the composite particle.
 7. The composite particle ofclaim 1, wherein the skeleton matrix material is in the form of amonolithic particle.
 8. The composite particle of claim 1, wherein theskeleton matrix material comprises carbon.
 9. The composite particle ofclaim 8, wherein the skeleton matrix material further comprises at leastone non-carbon material.
 10. The composite particle of claim 9, whereinthe at least one non-carbon material comprises nitrogen.
 11. Thecomposite particle of claim 1, wherein the metal of the mixturecomprises Fe, Zn, Cu, Cd, Sb, Ni, Pb, Bi or Sn.
 12. The compositeparticle of claim 11, wherein the metal of the mixture comprises atleast Cu.
 13. The composite particle of claim 1, wherein the conformalLi-ion permeable shell is a composite material that comprises at leasttwo components.
 14. The composite particle of claim 1, wherein theconformal Li-ion permeable shell comprises carbon.
 15. The compositeparticle of claim 1, wherein the composite particle further comprisesone or more functional groups forming a coating on the conformal Li-ionpermeable shell.
 16. The composite particle of claim 1, wherein skeletonmatrix material comprises porous carbon characterized by a Brunauer,Emmett and Teller (BET) specific surface area above 1000 m²/g.
 17. Thecomposite particle of claim 16, wherein the BET specific surface area isless than about 2630 m²/g.
 18. A Li or Li-ion battery, comprising: anodeand cathode electrodes, wherein the cathode electrode comprises aplurality of composite particles, each of the plurality of compositeparticles being an instance of the composite particle of claim 1; anelectrolyte ionically coupling the anode and cathode electrodes; and aseparator electrically separating the anode and cathode electrodes;wherein: the mixture is capable of storing and releasing Li-ions of theelectrolyte; and the conformal Li-ion permeable shell protects themixture from interaction with the electrolyte.
 19. The compositeparticle of claim 1, wherein the conformal Li-ion permeable shellcomprises one or more cracks.